Diana Deutsch is a professor of psychology at the University of San Diego, and a master at manipulating people's ears. While researching quirks in the way humans hear things, she has invented many auditory illusions. Here are some of the weirdest, and why they work.

Diana Deutsch was born in Britain, but came to the United States, apparently to mess with people's heads. Supposedly she has been studying things like how memory and music work, and how people manage to have perfect pitch. Somewhere along the line, though, she's worked out the many ways we aren't actually hearing what we think we're hearing, and she has taken advantage of it. The brain can be fooled by the ears just as it can be fooled by the eyes. The main difference is, we're quicker to realize that our eyes are fooling us.

Some Familiar Illusions

Not all the illusions that Deutsch has come up with over the years are entirely alien to us. Some are shocking because they're experiences all of us have shared without really knowing we shared them. For example, many of us have been in a group of people, and heard musical notes from another room. Some of us heard the notes as ascending, while another person in our group heard them as descending. Two people can disagree about this not because one of them is tone deaf, but because the tones are ambiguous. Deutsch explains:

The basic pattern that produces this illusion consists of two successively presented tones that are related by a half-octave (an interval known as a tritone). The composition of the tones is such that their note names are clearly defined, but they are ambiguous with respect to which octave they are in. So, for example, one tone might clearly be a C, but in principle it could be Middle C, or the C an octave above, or the C an octave below. So when listeners are asked to judge whether a pair of such tones forms an ascending or descending pattern, there is literally no correct answer.

An even more common experience is what Deutsch calls the speech-to-song illusion. Everyone in the world has heard a record or CD skip, or a computer freeze. If someone is speaking - not singing - while this freeze occurs, we often hear the same few words repeated again and again. The more we hear them, the more musical the words sound. The cadence and pitch of the spoken words becomes indistinguishable from a few repeated notes of music. True, it sounds like annoying music, but does sound like music.

After we deal with our technical problem, we replay the speech that we were listening to. It all sounds like a regular speech, until we get to that one group of words that we heard over and over. For a few seconds, it sounds like the person speaking has burst into song. Our brains can't stop hearing that often-repeated phrase as music.

And Then It Gets Weird

And then there are the less familiar illusions. We tend not to notice these illusions as we go through life, possibly because they're rare, and possibly because we're very unlikely to realize that we've been fooled. The key to many of them is what's known as the octave illusion or Deutsch's illusion. Deutsch found that the brain gets very confused when sounds are played alternately between one ear and the other. For example, sit a person down with a pair of headphones. The right headphone will play a high note while the left headphone will play a note one octave lower. The notes will switch back and forth between the headphones, but the person won't hear that switch between ears. They'll hear one low note in one ear, and then one high note in the other ear, and then back to the lower note again. Although they're hearing both sounds in both ears, they think they're hearing one sound at a time in each ear. That's just a kind of "hack" for the auditory system. Once you figure out this basic concept, there are all kinds of variations, many of which Deutsch invented.

For example, this is Deutsch's scale illusion. (You'll need headphones for it.) The first part of the video sounds like simultaneous ascending and descending scales, each being played into one of your ears. Then the sound drops out of one of the sides of the headphones, and you can hear the musical chaos of the other. Neither side of your headphones is playing a scale. The speaker over your right ear plays the first note of an ascending scale, the speaker on your left ear plays the next note, and the speaker on the right plays the note after that. Your brain mixes them together and you hear something completely different from what is being played.

There's also the glissando illusion. It's the same set-up, with the tones alternating between the two ears, but this time one is a continuous note, and the other is a glissando, a rising and falling siren-like a tone. The low tone will sound like it should - alternating from ear to ear - while the glissando note will seem to sound in both ears, or even move around randomly. There's the cambiata illusion, with the same set up as the Deutsch illusion except one ear will hear high notes of a tune and the other will low notes. Which ear will hear the notes depends, to a certain extent, on handedness. Right-handed people will most likely hear the high notes on the right, while left-handed people may hear the notes on the right or on the left.

And then there is the phantom words illusion. Again, the set up is like the Deutsch illusion. Two speakers play two sounds, the sounds alternating between the left and right speaker. The sounds are not words, but seem familiar. Once the sounds are played long enough, people begin to hear words in them. Generally, the sounds form words that are in the hearer's native language, whatever it might be. Many people find that they hear things on subjects that have been on their minds lately. The brain is given a wide leeway, and cobbles together all kinds of words. The words even change over time. I heard "no way." I'm not sure what that says about me, but in my defense, this illusion works best if it's heard on a stereo and not headphones.